Add j721s2 High Security EVM defconfig.
These configs are same as for the non-secure part, except for:
CONFIG_TI_SECURE_DEVICE option set to 'y'
CONFIG_FIT_IMAGE_POST_PROCESS option set to 'y'
CONFIG_SPL_FIT_IMAGE_POST_PROCESS option set to 'y'
CONFIG_BOOTCOMMAND uses FIT images for booting
Signed-off-by: Jayesh Choudhary <j-choudhary@ti.com>